The AI agrees with the suggested answer.
The recommended answer is D. DTMF was not negotiated on the call.
Reasoning: Based on the SDP offer/answer exchange, the initial INVITE message proposes payload type 110 for telephone-event (DTMF). However, the 200 OK response only includes G.729, and excludes payload type 110. This indicates that DTMF negotiation was not successful as the responder did not accept the offered DTMF payload type. Consequently, when users press keys, no DTMF signals are being transmitted or recognized because it wasn't negotiated during the call setup.
Why other options are incorrect:

A. DTMF was negotiated properly in these messages: This is incorrect because the 200 OK response does not include the DTMF payload type offered in the INVITE.
B. G.729 rather than G.711ulaw was negotiated: While the codec choice impacts voice quality, it doesn't directly prevent DTMF transmission if DTMF was negotiated as a separate payload.
C. Payload type 110 was negotiated rather than type 101: This is incorrect. Payload 110 was offered but not accepted. Although payload type 101 is commonly used for DTMF, the key point is that no DTMF payload was agreed upon in the final SDP.